Work vehicles such as excavators, backhoes, front shovels, and the like are used for excavation work. These excavating machines have work implements which consist of boom, stick and bucket linkages. The boom is pivotally attached to the excavating machine at one end, and to its other end is pivotally attached a stick. The bucket is pivotally attached to the free end of the stick. Each work implement linkage is controllably actuated by at least one hydraulic cylinder for movement in a vertical plane. Additionally, the work implement is transversely moveable relative to the machine. An operator typically manipulates the work implement to perform a sequence of distinct functions which constitute a complete excavation work cycle.
In a typical work cycle, the operator first positions the work implement at a trench location, and extends the work implement downward until the bucket penetrates the soil. Then the operator executes a digging stroke which brings the bucket toward the excavating machine until the stick is nearly fully retracted. The operator subsequently curls the bucket to capture the soil. To dump the captured load the operator raises the work implement, swings it transversely to a specified dump location, and releases the soil by extending the stick and uncurling the bucket. The work implement is then returned to the trench location to begin the work cycle again. In the following discussion, the above operations are referred to respectively as boom-down-into-trench, dig-stroke, capture-load, swing-to-dump, dump-load, and return-to-trench.
The earthmoving industry has an increasing desire to automate the work cycle of an excavating machine for several reasons. Unlike a human operator, an automated excavating machine remains consistently productive regardless of environmental conditions and prolonged work hours. The automated excavating machine is ideal for applications where conditions are dangerous and unsuitable for humans. An automated machine also enables more accurate excavation with regards to, for example, the trench depth and trench bottom slope, and the added ability to restrict digging in a predefined three dimensional area to avoid destroying utility lines or pipes.
Recent developments have produced a number of machines capable only of automating one or two functions of the excavation work cycle. One such example is described in U.S. Pat. No. 4,377,043 issued power shovel capable of returning a bucket to an original starting position after the operator manually dumps the load. Inui's system does not automate the dig-stroke, capture-load, swing-to-dump, dump-load, and return-to-trench portions of the work cycle.
To excavate and remove soil efficiently, it is desirable to obtain a heaped bucket when digging. The operator must dig and load the soil aggressively and yet simultaneously avoid stalling the hydraulic actuating system of the machine. Experienced operators anticipate stalling by "listening" to the hydraulic system, which emits a telltale noise when overloaded. However, this method has become unreliable with the quieter hydraulic systems of today. An automated excavating machine can anticipate stalling by sensing forces exerted on the work implement, and can take steps to relieve the overload and prevent stalling.
An excavation control apparatus described in Japanese Patent Publication No. Sho 61-9453 and published on Mar. 24, 1986 provides for detect relieving overload conditions encountered during excavation. Once an overload on the work implement is detected, the control apparatus attempts to relieve it by raising the boom for a fixed period of time. This scheme does not relieve all possible overloading conditions encountered during excavation. For example, when the bucket is caught under an obstacle, raising the boom exacerbates the problem. Because the work implement forces are not monitored at this time, the increased force on the stuck work implement is not detected and the boom cylinder hydraulic system may stall as a result. This control apparatus only performs the dig-stroke and capture-load functions of the work cycle.
The present invention automates the work cycle of an excavating machine and is directed to overcoming one or more of the problems as set forth above.